Vidic, 26, needs a more detailed examination this morning with United's medical staff resigned to the fact that he has suffered ligament damage.
The centre half looks to be out for a couple of weeks at least and perhaps will not play again this term.
It would be a big blow to United's chances of winning the Barclays Premier League and the Champions League and would bring back painful memories of last season when a sudden rush of injuries left Ferguson's side unable to get past the last four in Europe.
A United source said last night: "Nemanja came in this morning but could hardly walk. His knee is swollen and he did not look very happy after his first scan."